Sri Lanka President Mahinda Rajapaksa ordered a probe on Friday into the mass disappearances during the country's civil war.
"The president has directed his secretary to take necessary measures to institute a commission to look into disappearances during the conflict period," said presidential spokesman Mohan Samaranayake.
"It is the (president's) secretary who will decide on the terms of reference, who the members will be, the time frame and so on."
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The president made the decision after being pressured by the United Nations and the West to address alleged rights abuses during the nearly three-decade war, which came to an end in 2009.
A UN investigation indicated that government troops might have killed as many as 40,000 minority ethnic Tamil civilians in the final months of the Sri Lankan war. Rajapaksa's government had initially denied any civilian deaths, but later agreed to look into alleged abuses.
Four years after the conflict ended, hundreds of mostly Tamils are still missing.
UN High Commissioner for Human Rights Navi Pillay is set to visit Sri Lanka next month and the Commonwealth Heads of Government Meeting will take place in the country in November.
But former diplomat and now independent analyst Dayan Jayan Jayatilleka has said that Rajapaksa's decision to open an inquiry was not to respond to UN queries, but to pressure from abroad, and that it needs to be done by an independent body to be credible.
"The announcement is clearly coincident with the impending visit of United Nations High Commissioner for Human Rights Navi Pillay," he told Reuters.
"The move may lack credibility unless the probe is conducted by respected, independent personalities. It cannot be an in-house matter. For instance, it can't be coordinated by the secretary to the president. The credibility is important."